EAVESDROPPING ON SUBCARRIER TRANSMISSIONS


A few weeks ago, I posed a question about listening to
SCA transmissions on the FM commercial broadcast band.
An article in Monitoring_Times claimed that connecting
a vlf receiver to an FM broadcast receiver would permit
SCA detection.

Necessity being the mother of invention, a way was
found to confuse the ICOM R71A into tuning below 100
kHz1, and after receiving substantial inspiration from
Will Martin, Phil Karn, and others I now can report
success!

With the R71A in the FM mode, tuned to 67 kHz, I con-
nected the ICOM's vlf antenna input through a 0.1 ufd
capacitor2 to the earphone jack of a $16 General Elec-
tric AM/FM portable radio, and can now listen in on the
world of SCA!

In the first few minutes of tuning around, I've heard
the Physicians' Network, Muzak, commodity reports, and
several data transmissions.

I built a simple SCA interface, consisting of a capaci-
tor and resistor, into a plastic film canister.


+-----------------------+
| AM/FM radio |
| |
| earphone jack |
| (ring) (tip) |
+-----------------------+
| |
+--/|/|/|/--+
| |
| 100 ohm |
| 1 watt |
| |
| |
| -----
| ----- 0.1 ufd
| |
+-----------------------+
| (gnd) |
| antenna jack |
| |
| ICOM R71A |
+-----------------------+


Most activity is heard with AM/FM radio tuned to the FM
broadcast band, and the R71A set to 67 kHz FM. Aside
from the Muzak, commodity reporting, and the Physici-
ans' Network, I also heard an announcer reading from
Popular_Communications on CRIS, the Chicago Radioland
Information Service. This service carries programming
of interest to the handicapped.

With the AM/FM radio tuned to the AM broadcast band,
and the R71A set to 60 kHz AM, I can hear what seems to
be stereo subcarriers on AM broadcast stations claiming
to transmit in AM stereo.

__________

1. Parnass, Bob, "Trick the ICOM R71A below 100 kHz", to be
published in Monitoring_Times.

2. The capacitor is needed with the GE radio I used to
block DC.





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